15.7 Exhausts & Thrust Reversers Flashcards
What is the main purpose of a jet engine exhaust?
To accelerate the airflow coming from the turbines to efficiently make thrust
What is the main purpose of the exhaust on a turboprop engine?
To safely guide the exhaust gases overboard
What is the purpose of the exhaust cone?
It guides the flow and prevents reverse flow of the air.
It also prevents excessive turbulence of the gases.
What type of aircraft use long exhaust ducts?
On older aircraft where the engine is mounted directly under the wing
What is the advantage of using a common exhaust nozzle?
It mixes the bypass and core air (slowing the velocity of the exhaust gases) which gives a higher propulsion efficiency
What is the disadvantage of using a common exhaust nozzle?
Added weight of the engine
What is the advantage of using a mixer on the exhaust of a jet engine?
It increases the propulsion efficiency by mixing the air inside a common exhaust nozzle.
They aren’t normally used anymore as they add weight
How is optimum thrust achieved in relation to the exhaust gases?
All of the exhaust gases must be fully expanded inside the exhaust nozzle which guarantees a Laminar vortex-free and axial orientated exhaust gas flow
How does the pressure ratio change in relation to the engine speed?
Increasing the engine speed increases the pressure ratio (a higher airflow through the exhaust nozzle)
In a convergent exhaust nozzle, what pressure ratio permits maximum airflow?
A pressure ratio of 2:1
What is a choked condition of the exhaust nozzle?
When the engine speed is increased above the maximum airflow (pressure ratio 2:1) chocking occurs as not all of the air can escape quickly enough.
At the smallest part of the exhaust, the airflow equals the speed of sound.
What are C-D nozzles?
Convergent-Divergent nozzles, used mainly on aircraft which fly faster than the speed of sound. They have a variable design
Why must high exhaust gases be avoided on a commercial turbofan or turbojet engine?
- It decreases the propulsion efficiency as it creates turbulent airflow when mixed with the ambient airflow
- It creates a loud exhaust noise
What is the unit commonly used to express aircraft noise level?
Effective Perceived Noise deciBel (EPNdB)
What does a turbofan engine’s noise depend on?
The bypass ratio
What is used to provide noise suppression of the internal noise generated?
Acoustic linings
What is a disadvantage of using acoustic linings to suppress internal noise?
Added weight to the engine
What is used to suppress the external noise of a low-bypass engine or turbojet engine generated?
Hush kits
- lobe type nozzles attached to the exhaust which mixes both core and ambient air (or small amount of bypass air)
What is the usual deflection of air when using reverse thrust compared to the ideal?
Usual deflection of exhaust gases = 45-60 degrees
Ideal deflection = 180 degrees
On a turbofan engine, what air is deflected for reverse thrust?
Bypass (fan) air only
What are the four different types of thrust reversers?
- Clamshell
- Bucket-type
- Translating sleeves (cascade type)
- Pivot-type
How can the cascade type translating sleeve thrust reverser be operated?
Pneumatically or Hydraulically
What is the modern turbofan engine thrust reverser also known as?
Fan reverser
What were the disadvantages to the older hot core reversers?
- inefficient
- very extensive maintenance needed due to the hot temperatures
What are the disadvantages of using thrust reversers?
- They can whirl up FOD into the intake
- They are less efficient at low speeds (ingesting own airflow).
- The compressor can stall due to the deployment of reverse thrust
- The reversers can cause a cushion of air between the lower wing and the ground, reducing the efficiency of the brakes and landing gear and contact with the runway.
How are the disadvantages to thrust reverse minimised?
The airflow is matched to engine position and deflected at specific angles for the safest operation. This is completed by the cascade vanes.
What must be done to an aircraft with one faulty thrust reverser?
It must be deactivated properly so that it does not deploy unintentionally. The aircraft can still be dispatched
What are the three main subsystems of a thrust reverser system?
1) The control system
2) The actuator system
3) The airflow deflection system
How are the thrust reversers protected during flight so that they cannot deploy?
The air-ground logic
Some aircraft can deploy the thrust reverser immediately prior to landing. How does this system operate?
Using the radio altimeter - deployable at less than 10 feet
This is an advantage for short runway landings
What is the purpose of the thrust reverser interlock system?
It makes sure that the engine power can only be increased after the reverser is fully deployed
Why are hydraulic actuators on the translating sleeve type thrust reverser more complicated?
They have to be synchronised
What type of thrust reverser system could you find a pneumatic actuation system instead of a hydraulic actuation system?
Cascade type translating sleeve thrust reversers can be pneumatic or hydraulically operated
What type of thrust reverser is usually always pneumatically operated?
The older clam-shell thrust reverser (via HP compressor)
What is the purpose of the locking system for the thrust reversers?
It prevents the thrust reversers deploying unintentionally and them safe in the stowed position.
How is a pneumatic thrust reverser actuator locked out?
They have brakes at the air motor as the locking device
If a cascade vane from the right hand cowling which is an upward pointing segment is placed on the left hand side, where does the vane direct the airflow?
If placed on the left hand cowling, it would point downwards
What indication would you find in the cockpit that tells the crew that the thrust reverser isn’t safe in its stowed position?
A “reverse unlock” message in amber
What would a green thrust reverse light mean in the cockpit?
That the reverser is fully deployed and the interlock has been released so that you can increase the engine power
